1. Field of the Invention
The present invention relates to a pulse transformer, and more particularly relates to a surface-mount pulse transformer configured by using a drum core and a plate core.
2. Description of Related Art
When a device such as a personal computer is connected to a network such as a LAN or a telephone network, it is necessary to protect the device from the entry of ESD (ElectroStatic Discharge) and high voltage via a cable. Therefore, a pulse transformer is used in a connector that constitutes a connection point between the cable and the device.
In recent years, as the pulse transformer described above, a surface-mount pulse transformer suitable for high-density mounting is frequently used. The surface-mount pulse transformer is configured by using a drum core and a plate core. The drum core is a magnetic body, and includes a winding core portion, and a pair of flange portions that are formed at both ends of the winding core portion, where the winding core portion and the pair of flange portions are formed integrally with each other. Four wires that constitute a coil are wound around the winding core portion. These wires are connected to their respective terminal electrodes formed on each bottom surface of the pair of flange portions. The plate core is a magnetic body fixed to each top surface of the pair of flange portions. The plate core and the drum core constitute a closed magnetic path therebetween. Japanese Patent Application Laid-open No. 2009-302321 discloses an example of the surface-mount pulse transformer as described above.
According to the American National Standard Institute Standards “ANSI X3.263: 1995 (TP-PMD)” incorporated into Chapter 25 of the Ethernet Alliance Standards “IEEE-802.3”, a pulse transformer used for 100Base-TX is required to achieve an inductance of 350 μH or higher under a bias current of 0 mA to 8 mA. This inductance value is very large for a small-sized pulse transformer. In order to achieve this, various improvements are needed. The technique described in Japanese Patent Application Laid-open No. 2009-302321 is one of the improvements, and achieves the above standard value by using a plate core and a drum core that have undergone mirror finishing on their contact surfaces to reduce the magnetic resistance in a magnetic path.
The principles of obtaining the inductance that satisfies the above standard value by the technique in Japanese Patent Application Laid-open No. 2009-302321 are explained below with reference to FIG. 12. FIG. 12 shows the imaginary relationship between a bias current and an inductance, which is created by the inventors of the present application, and does not show the actual measurement results.
A curved line “a” in FIG. 12 shows an example of the relationship between an inductance and a bias current in a pulse transformer in which mirror finishing is not performed on the contact surfaces of a plate core and a drum core, and an adhesive is applied to the entire contact surfaces. As shown in FIG. 12, the inductance in this example is lower than 350 μH regardless of the value of the bias current. That is, the inductance does not satisfy the above standard value at all. This is because grinding is not performed on the plate core and the drum core, and therefore there is a gap created therebetween, and moreover this gap is even enlarged by the thickness of the adhesive. If there is the gap, the magnetic resistance is increased accordingly, which results in a reduction in the inductance.
A curved line “b” in FIG. 12 shows an example of the relationship between an inductance and a bias current in a pulse transformer in which mirror finishing is performed on the contact surfaces of a plate core and a drum core, and an adhesive is applied to the entire contact surfaces. Although the inductance in this example is comparatively higher than that in the example of the curved line “a”, it is still lower than 350 μH regardless of the value of the bias current. This is because a gap is created between the plate core and the drum core according to the thickness of the adhesive.
In contrast to these examples, a curved line “c” in FIG. 12 shows the relationship between an inductance and a bias current in the pulse transformer in Japanese Patent Application Laid-open No. 2009-302321. In the pulse transformer in Japanese Patent Application Laid-open No. 2009-302321, a groove is provided on the contact surface of the drum core to apply the adhesive only to the interior of the groove, and moreover mirror finishing is performed on the contact surfaces of the plate core and the drum core. Therefore, the plate core and the drum core come into close contact with each other except the groove portion. This suppresses the magnetic resistance on the contact surfaces to a low level in the poise transformer in Japanese Patent Application Laid-open No. 2009-302321. As a result, the inductance that exceeds 350 μH is achieved under a bias current of 0 mA to 8 mA as shown in FIG. 12.
However, the technique in Japanese Patent Application Laid-open No. 2009-302321 has a problem that, while the pulse transformer with a size (4.5 mm×3.2 mm×2.9 mm) described in Japanese Patent Application Laid-open No. 2009-302321 can achieve the inductance that satisfies the above standard value, the pulse transformer with a smaller size (for example, 3.3 mm×3.3 mm×2.7 mm) cannot obtain a sufficient inductance particularly when the bias current is high. This problem is explained below in detail.
As shown in FIG. 12, the inductance along the curved line “c” is decreased as the bias current is increased within the range between 0 mA and 8 mA. This is because, as a result of bringing the plate core and the drum core into close contact with each other by the mirror finishing, magnetic saturation is more likely to occur in the pulse transformer in Japanese Patent Application Laid-open No. 2009-302321. As the bias current is increased, the amount of magnetic saturation becomes larger, and accordingly the inductance is reduced. Therefore, in the case of bringing the plate core and the drum core into close contact with each other to increase the inductance, it is necessary to consider this inductance reduction in designing a pulse transformer.
One of the important factors for designing such a pulse transformer is the inductance obtained when the bias current is 0 mA (hereinafter, “inductance initial value”). Assuming that the inductance initial value is sufficiently large, even when the inductance is decreased by magnetic saturation inversely proportional to the increase in bias current, this inductance can still be maintained at 350 μH or higher under the bias current of 8 mA as shown by the curved line “c” in FIG. 12.
Provided that the contact surfaces of the plate core and the drum core are under the same conditions, the inductance initial value becomes larger as the cross-sectional area of the magnetic path in the contact-surface portion becomes larger. In the pulse transformer in Japanese Patent Application Laid-open No. 2009-302321, the groove portion does not function as a magnetic path. However, the size of the pulse transformer is large originally enough to ensure a sufficiently large cross-sectional area of the magnetic path in the contact-surface portion. Therefore, as shown by the curved line “c” in FIG. 12, the inductance initial value is sufficiently large (which can maintain the inductance at 350 μH or higher under the bias current of 8 mA).
In contrast to this, in a pulse transformer with a smaller size of 3.3 mm×3.3 mm×2.7 mm, although assuming that the need for adhesion is ignored, and thus an adhesive filling groove is not provided, it is still difficult to increase the cross-sectional area of the magnetic path in the contact-surface portion to such a degree as to obtain an inductance initial value large enough to compensate for a decrease in the inductance due to magnetic saturation. A curved line “d” in FIG. 12 shows an example of the pulse transformer with the size of 3.3 mm×3.3 mm×2.7 mm. This example is a hypothetical example in which an adhesive filling groove is not provided, and the inductance is below 350 μH under the bias current of 8 mA. As described above, in the technique in Japanese Patent Application Laid-open No. 2009-302321, the pulse transformer with a smaller size cannot satisfy the standard inductance value in some cases, and is required to be improved.